Purpose: To investigate the relationship between classification based on simple radiographic findings and arthroscopic findings of the cartilage lesions in medial degenerative arthritis of the ankle joint. Materials and Methods: We studied 41 ankles of 36 patients with asymmetrical narrowing of the medial joint space. Degenerative arthritis following fracture and those with generalized arthritic disease were excluded, but those with a history of ankle sprain were included. Standing radiographs of all patients were graded according to the Takakura classification and the Kellgren-Lawrence (K/L) classification. Arthroscopic findings were classified according to the depth, width, and anteroposterior dimension of articular cartilage damage. Results: According to the Takakura classification, 29 ankles were classified as stage II, 7 cases as stage IIIA and 2 cases as stage IIIB. According to our classification of arthroscopic findings of 29 ankles in stage II, 1 ankle was graded as Grade I, 3 ankles as grade II, 10 ankles as grade III, and 15 ankles as grade IV. Spearman correlation coefficient between Takakura classification and arthroscopic classification was 0.342 (P=0.028), and coefficient between K/L classification and arthroscopic classification was 0.480 (P=0.001). Conclusion: Degenerative changes of the articular cartilage are more advanced than radiographic findings in many patients with ankle degenerative arthritis with asymmetrical narrowing of medial joint space. Therefore, we conclude that more aggressive effort should be made for correct diagnosis and treatment of degenerative arthritis.